[Melbourne-pm] basic regex - char array vs "pipe brackets"

[Myf White]

On Thu, Mar 5, 2009 at 4:25 PM, Toby Corkindale <
toby.corkindale at strategicdata.com.au> wrote:

> wigs at stirfried.org wrote:
>
>> On Thu, Mar 05, 2009 at 03:10:47PM +1100, Christopher Short wrote:
>>
>>> I saw this regex in someone else's code (to check validity of an id
>>> field)
>>> /^(\w|\@|\.|\-)+$/
>>>
>>> and decided that it wouldn't work properly, that what they'd meant was
>>> the character array
>>> /^[\w\@\.\-]+$/
>>>
>>
>>  Luckily I chucked it into Regex Coach and found both of them worked
>>> just as well.
>>> Thing is, when I look at
>>> /^(\w|\@|\.|\-)+$/
>>>
>>
>> The '+' here means one or more repetion of the previous regex, not
>> whatever it happens to match.
>>
>>  Turns out the long-term perl developer who wrote that code always uses
>>> "pipe brackets" instead of character arrays.
>>> Do they really function identically?
>>>
>>
>> The biggest difference between character arrays and the parens grouping
>> is that the character array allows for a compact specification of ranges,
>> eg:
>>
>>  [a-z] is the same as
>> (?:a|b|c|d|e|f|g|h|i|j|k|l|m|n|o|p|q|r|s|t|u|v|w|x|y|z)
>>
>> As such, the hyphen has special meaning within the character set,
>> this is often a source of surprise to some perl developers when the
>> regex doesn't do what they expect.
>>
>
> Although it's useful to remember that if the first character in a character
> array is a hyphen, then it is matched literally, ie:
>
> 'foo-bar' =~ /[-a-z]/ ; # is true
>
>
> tjc.


Actually, that regex is only matching the first character, ie the 'f', so
would be true without the initial hypen. I think you mean that every
character in the string is in the character class, so
'foo-bar' =~ /[-a-z]{7}/ ; # is true

You can also put the hyphen last in the character classe, ie
'foo-bar' =~ /[a-z-]{7}/ ; # is also true

However, I thinks it's better to always put it first, because otherwise it's
very easy to miss.
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